1. Field of the Invention
The present invention relates to Dynamic Random Accessible Memory (DRAM). More particularly, this invention relates to DRAM fabricated using a conventional logic process. This invention further relates to the on-chip generation of precision voltages for the operation of DRAM embedded or fabricated using a conventional logic process.
2. Related Art
FIG. 1A is a schematic diagram of a conventional DRAM cell 100 that is fabricated using a conventional logic process. FIG. 1B is a cross sectional view of DRAM cell 100. As used herein, a conventional logic process is defined as a semiconductor fabrication process that uses only one layer of polysilicon and provides for either a single-well or twin-well structure. DRAM cell 100 consists of a p-channel MOS access transistor 1 having a gate terminal 9 connected to word line 3, a drain terminal 17 connected to bit line 5, and a source terminal 18 connected to the gate 11 of a p-channel MOS transistor 2. P-channel transistor 2 is configured to operate as a charge storage capacitor. The source and drain 19 of transistor 2 are commonly connected. The source, drain and channel of transistor 2 are connected to receive a fixed plate bias voltage V.sub.pp. The V.sub.pp voltage is a positive boosted voltage that is higher than the positive supply voltage V.sub.dd by more than a transistor threshold voltage V.sub.t.
As used herein, the electrode of the charge storage capacitor is defined as the node coupled to the access transistor, and the counter-electrode of the charge storage capacitor is defined as the node coupled to receive a fixed plate bias voltage. Thus, in DRAM cell 100, the gate 11 of transistor 2 forms the electrode of the charge storage capacitor, and the channel region of transistor 2 forms the counter-electrode of the charge storage capacitor.
To improve soft-error-rate sensitivity of DRAM cell 100, the cell is fabricated in an n-well region 14 which is located in a p-type substrate 8. To minimize the sub-threshold leakage of access transistor 1, n-well 14 is biased at the V.sub.pp voltage (at n-type contact region 21). However, such a well bias increases the junction leakage. As a result, the bias voltage of n-well 14 is selected such that the sub-threshold leakage is reduced without significantly increasing the junction leakage. When storing charge in the storage capacitor, bit line 5 is brought to the appropriate level (i.e., V.sub.dd or V.sub.SS) and word line 3 is activated to turn on access transistor 1. As a result, the electrode of the storage capacitor is charged. To maximize the stored charge, word line 3 is required to be driven to a negative boosted voltage V.sub.bb that is lower than the supply voltage V.sub.SS minus the absolute value of the threshold voltage (V.sub.tp) of access transistor 1.
In the data retention state, access transistor 1 is turned off by driving word line 3 to the V.sub.dd supply voltage. To maximize the charge storage of the capacitor, the counter electrode is biased at the positive boosted voltage V.sub.pp. The plate voltage V.sub.pp is limited by the oxide breakdown voltage of the transistor 2 forming the charge storage capacitor.
DRAM cell 100 and its variations are documented in U.S. Pat. No. 5,600,598, entitled "Memory Cell and Wordline Driver For Embedded DRAM in ASIC Process," by K. Skjaveland, R. Township, P. Gillingham (hereinafter referred to as "Skjaveland et al."), and "A 768k Embedded DRAM for 1.244 Gb.s ATM Switch in a 0.8 um Logic Process," P. Gillingham, B. Hold, I. Mes, C. O'Connell, P. Schofield, K. Skjaveland, R. Torrance, T. Wojcicki, H. Chow, Digest of ISSCC, 1996, pp. 262-263 (hereinafter referred to as "Gillingham et al.). Both Skjaveland et al. and Gillingham et al. describe memory cells that are contained in an n-well formed in a p-type substrate.
FIG. 2 is a schematic diagram of a word line control circuit 200 including a word line driver circuit 201 and a word line boost generator 202 described by Gillingham et al.. Word line control circuit 200 includes p-channel transistors 211-217, inverters 221-229, NAND gates 231-232 and NOR gate 241, which are connected as illustrated. Word line driver 201 includes p-channel pull up transistor 211, which enables an associated word line to be pulled up to the V.sub.dd supply voltage. P-channel pull down transistors 212-217 are provided so that the word line can be boosted down to a negative voltage (i.e., -1.5 V) substantially below the negative supply voltage V.sub.SS. However, the p-channel pull down transistors 212-217 have a drive capability much smaller (approximately half) than an NMOS transistor of similar size. As a result, the word line turn on of Gillingham et al. is relatively slow (&gt;10 ns). Furthermore, in the data retention state, word line driver 201 only drives the word line to the V.sub.dd supply voltage. As a result, the sub-threshold leakage of the access transistor in the memory cells may not be adequately suppressed.
DRAM cells similar to DRAM cell 100 have also been formed using n-channel transistors fabricated in a p-type well region. To maximize stored charge in such n-channel DRAM cells during memory cell access, the associated word line is driven to a voltage higher than the supply voltage V.sub.dd plus the absolute value of the threshold voltage (V.sub.tn) of the access transistor. In the data retention state, the n-channel access transistor is turned off by driving the word line to V.sub.SS supply voltage (0 Volts) To maximize the charge storage of the capacitor in an n-channel DRAM cell, the counter electrode is biased at a plate voltage V.sub.bb that is lower than the V.sub.SS supply voltage.
A prior art scheme using n-channel DRAM cells includes the one described by Hashimoto et al. in "An Embedded DRAM Module using a Dual Sense Amplifier Architecture in a Logic Process", 1997 IEEE International Solid-State Circuits Conference, pp. 64-65 and 431. A p-type substrate is used, such that the memory cells are directly in contact with the substrate and are not isolated by any well structure. In the described design, substrate bias is not permitted. Moreover, application of a negative voltage to the word line is not applicable to ASICs that restrict substrate biasing to be zero. Consequently, the architecture achieves a negative gate-to-source voltage (V.sub.gs) by limiting bit line swing. The negative V.sub.gs voltage reduces sub-threshold leakage in the memory cells. Hashimoto et al. fails to describe the structure of the word line driver.
It would therefore be desirable to have a word line driver circuit that improves the leakage currents in DRAM cells fabricated using a conventional logic process. Moreover, it would be desirable to have improved methods for biasing DRAM cells fabricated using a conventional logic process.